If it's not a Rectangle it's either a Trapezoid or Parallelogram...
We know....that is why I was asking if the opposing sides were the exact same lengths....
Anyway....assuming the Front/Back is the same width....and left/right sides are exactly the same (Parallelogram), then I'm about 1/2" off.
In other words.....if the OP sheaths his roof using one wall as the 'line'...when he gets to the other side....the sheathing will be about 1/2" off relative to the wall.
This is pretty much in range with what Git came up with.
The variable....if one wall is not exactly the same length as the opposing wall, then we have a trapezoid....which changes things a little.
But....at the end of the day....the difference is so small, and the garage is so large....the only place the issue is going to show up is on the roof....and the error is so small, it will be hidden by the typical framing tolerances (translation...close is good enough).
Let me state it this way.....in the framing world, that garage is pretty f'ing square.
Now.....if you want to talk about 'not square'....then lets talk about my neighbor (poor lady). She let a contractor talk her into converting her detached rear garage into a living space.
Not sure what was wrong with his chalk line....he added 20' to the side of the garage...if you follow the line of the back wall of the garage....the corner of the addition is about 8" off that line.
On a related note......besides being overcharged for sub-standard work...and him forging checks to the tune of about $20K...he did not have a contractors license...did not pull permits...and is now hiding in Mexico. She had to get 'real' contractor to pull off ALL the inside drywall so building and safety can inspect the electrical and plumbing.
You should see the roof....about a 1:12 pitch....and they used shingles.... no eaves...no drip edge....looks like a ******* stucco brick....